Showerhead electrode assembly with gas flow modification for extended electrode life

ABSTRACT

A showerhead electrode assembly for a plasma processing apparatus is provided. The showerhead electrode assembly includes a first member attached to a second member. The first and second members have first and second gas passages in fluid communication. When a process gas is flowed through the gas passages, a total pressure drop is generated across the first and second gas passages. A fraction of the total pressure drop across the second gas passages is greater than a fraction of the total pressure drop across the first gas passages.

This application is a divisional of U.S. patent application Ser. No.11/640,193, entitled SHOWERHEAD ELECTRODE ASSEMBLY WITH GAS FLOWMODIFICATION FOR EXTENDED ELECTRODE LIFE, filed on Dec. 18, 2006, theentire content of which is hereby incorporated by reference.

BACKGROUND

Plasma processing apparatuses are used to process substrates bytechniques including etching, physical vapor deposition (PVD), chemicalvapor deposition (CVD), ion implantation, and resist removal. One typeof plasma processing apparatus used in plasma processing includes areaction chamber containing upper and lower electrodes. An electricfield is established between the electrodes to excite a process gas intothe plasma state to process substrates in the reaction chamber.

SUMMARY

Showerhead electrode assemblies for a plasma processing apparatus areprovided. In an exemplary embodiment, the showerhead electrode assemblycomprises an electrode having a plurality of first gas passages and aplasma-exposed surface. A backing member is attached to the electrodeand has a plurality of second gas passages in fluid communication withthe first gas passages. One or more first plenums are formed in thebacking member and in fluid communication with the second gas passages.When a process gas is flowed through the first and second gas passages,a total pressure drop is generated across the first and second gaspassages. A fraction of the total pressure drop across the second gaspassages is greater than a fraction of the total pressure drop acrossthe first gas passages.

Another exemplary embodiment of the showerhead electrode assemblies fora plasma processing apparatus includes a silicon electrode with aplasma-exposed surface and a plurality of axially extending first gaspassages. A metallic backing member is attached to the electrode and hasa plurality of axially extending second gas passages in fluidcommunication with the first gas passages. One or more first plenums areformed in the metallic backing member and in fluid communication withthe second gas passages. When a process gas is flowed through the firstand second gas passages, a total pressure drop is generated across thefirst and second gas passages. A fraction of the total pressure dropacross the second gas passages is greater than a fraction of the totalpressure drop across the first gas passages.

Another exemplary embodiment of the showerhead electrode assemblies fora plasma processing apparatus, includes a first member having aplurality of first gas passages having a first portion and secondportion wider than the first portion. The first member has aplasma-exposed surface and the second portion is adjacent to theplasma-exposed surface. A second member is attached to the first surfaceof the first member, the second member having a plurality of second gaspassages in fluid communication with the first gas passages. When aprocess gas is flowed through the first and second gas passages, a totalpressure drop is generated across the first and second portions. Afraction of the total pressure drop across the second portion is greaterthan a fraction of the total pressure drop across the first portion.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a cross-sectional view of a portion of an embodimentof a showerhead electrode assembly and a substrate support for a plasmaprocessing apparatus.

FIG. 2A is an enlarged view of a portion of the showerhead assemblyshown in FIG. 1, depicting the thermal control plate and gas passages inthe backing member and upper electrode.

FIG. 2B shows the upper electrode structure of FIG. 2A with eroded gaspassages resulting from plasma exposure.

FIG. 3 illustrates a portion of an additional embodiment of a showerheadelectrode assembly and a substrate support for a plasma processingapparatus.

FIG. 4A is an enlarged view of a portion of the showerhead electrodeassembly shown in FIG. 3, depicting the thermal control plate and gaspassages in the backing member and upper electrode and a plenum betweenthe backing member and upper electrode.

FIG. 4B is a plan view of the portion of the showerhead electrodeassembly shown of FIG. 4A, depicting gas passages in backing member andupper electrode and a plenum between the backing member and upperelectrode.

FIG. 5A illustrates an additional embodiment of the showerhead electrodeassembly, including a thermal control plate and gas passages in thebacking member and upper electrode and plenums between the backingmember and upper electrode and between the thermal control plate andbacking member.

FIG. 5B is a plan view of the showerhead electrode assembly of FIG. 5A,depicting gas passages in the backing member and upper electrode andplenums between the backing member and upper electrode and between thethermal control plate and backing member.

FIG. 6 is a top view of the embodiment of the showerhead electrodeassembly shown in FIG. 4A, illustrating the arrangement of gas passagesand plenums in the backing member (broken lines) in relation to gaspassages in the upper electrode (solid lines).

FIG. 7 is a three-dimensional perspective view of a portion of theembodiment of the showerhead electrode assembly of FIG. 6, showing theupper electrode (solid lines) and gas passages and plenums of thebacking member (broken lines).

FIG. 8 shows a portion of an additional embodiment of the showerheadelectrode assembly, depicting a thermal control plate and gas passagesin the backing member and upper electrode, the gas passage in the upperelectrode having portions with different cross-sectional areas.

DETAILED DESCRIPTION

During the operation of a plasma etching or deposition system, aconstant mass flow rate of reactants (i.e., process gas) is typicallydelivered into the processing chamber to achieve a desirable etching ordeposition rate on the wafer surface (e.g., microns/minute of a thinfilm etched or deposited on the wafer surface). Localized variations inthe gas throughput from the gas distribution system into the processingchamber can result in non-uniform etching across the surface of thewafer. Accordingly, a gas distribution system that can introduce asubstantially constant gas throughput into the processing chamber duringplasma processing can be advantageous in maintaining uniform etching ordeposition.

The plasma etch conditions create significant ion bombardment of thesurfaces of the processing chamber that are exposed to the plasma. Thision bombardment, combined with plasma chemistries and/or etchbyproducts, can produce significant erosion, corrosion andcorrosion-erosion of the plasma-exposed surfaces of the processingchamber. As a result, surface materials are removed by physical and/orchemical attack, including erosion, corrosion and/or corrosion-erosion.This attack causes problems including short part lifetimes, increasedparts costs, particulate contamination, on-wafer transition metalcontamination and process drift.

Parts with relatively short lifetimes are commonly referred to as“consumables,” for example, silicon electrodes. If the consumable part'slifetime is short, then the cost of ownership is high. Erosion ofconsumables and other parts generates particulate contamination inplasma processing chambers. Silicon electrode assemblies used indielectric etch tools deteriorate after a large number of RF hours (timein hours during which radio frequency power is used to generate theplasma) are run using the electrode assemblies. Such used siliconelectrode assemblies exhibit etch rate drop and etch uniformity driftafter a large number of RF hours are run using the electrode assemblies.

When a silicon electrode assembly, such as a showerhead electrode, isexposed to a plasma environment, erosion, corrosion and/orcorrosion-erosion of the gas distribution passages occurs. Erosiontypically occurs at the edges of the gas distribution passages,resulting in a widening of the passages. As a result, the overallprocess gas throughput for every one of the gas distribution passagesincreases. Additionally, this widening of the passages can also cause adrift in the mixing ratios and/or velocity of the process gases. Becauseetching or deposition rates of films ideally requires a specific mass ofreactants from the process gas, variation in process gas throughput(i.e., volumetric gas flow rate), can locally alter the etching ordeposition rates of films. As a result, erosion of the gas distributionpassages can result in non-uniform etching across the wafer.

FIG. 1 illustrates an embodiment of a showerhead electrode assembly 10for a plasma processing apparatus in which semiconductor substrates,e.g., silicon wafers, are processed. The showerhead electrode assemblyis described, for example, in commonly-owned U.S. Patent ApplicationPublication No. 2005/0133160, which is incorporated herein by referencein its entirety. The showerhead electrode assembly 10 comprises ashowerhead electrode including an upper electrode 12, a backing member14 secured to the upper electrode 12, having a plasma-exposed bottomsurface 13 and a thermal control plate 16 provided on the backing member14. A substrate support 18 (only a portion of which is shown in FIG. 1)including a bottom electrode and optional electrostatic clampingelectrode is positioned beneath the upper electrode 12 in the plasmaprocessing chamber of the plasma processing apparatus. A substrate 20subjected to plasma processing is mechanically or electrostaticallyclamped on an upper support surface 22 of the substrate support 18.

The upper electrode 12 can be electrically grounded, or alternativelycan be powered, such as by a radio-frequency (RF) current source. In anembodiment, the upper electrode 12 is grounded, and power at one, two ormore frequencies is applied to the bottom electrode to generate plasmain the plasma processing chamber. For example, the bottom electrode canbe powered at frequencies of 2 MHz and 27 MHz by twoindependently-controlled radio-frequency power sources. After substrate20 has been processed, the supply of power to the bottom electrode isshut off to terminate plasma generation.

In the embodiment shown in FIG. 1, the upper electrode 12 of theshowerhead electrode includes an inner electrode member 24, and anoptional outer electrode member 26. The inner electrode member 24 istypically a cylindrical plate (e.g., a plate composed of single crystalsilicon or silicon carbide). The inner electrode member 24 can have adiameter smaller than, equal to, or larger than a wafer to be processed,e.g., a diameter up to 12 inches (300 mm) if the plate is made of singlecrystal silicon. In a preferred embodiment, the showerhead electrodeassembly 10 is large enough for processing large substrates, such assemiconductor wafers having a diameter of 300 mm or larger. For 300 mmwafers, the upper electrode 12 is up to 300 mm in diameter. However, theshowerhead electrode assembly 10 can be sized to process other wafersizes or substrates having a non-circular configuration.

In the illustrated embodiment, the inner electrode member 24 is widerthan the substrate 20. For processing 300 mm wafers, the outer electrodemember 26 is provided to expand the diameter of the upper electrode 12from, for example, about 15 inches to about 17 inches. The outerelectrode member 26 can be a continuous member (e.g., a continuouspoly-silicon ring), or a segmented member (e.g., including 2-6 separatesegments arranged in a ring configuration, such as multiple segmentscomposed of single crystal silicon). In embodiments of the upperelectrode 12 that include a multiple-segment, outer electrode member 26,the segments preferably have edges, which overlap each other to protectan underlying bonding material from exposure to plasma. The innerelectrode member 24 preferably includes multiple gas passages 28extending through and in correspondence with multiple gas passages 30formed in the backing member 14 for injecting process gas into a spacein a plasma processing chamber located between the upper electrode 12and the substrate support 18. The thermal control plate 16 includesmultiple plenums 31 to distribute process gases to the gas passages 28and 30 in the inner electrode member 24 and backing member 14,respectively.

Single crystal silicon is a preferred material for the inner electrodemember 24 and the outer electrode member 26. High-purity, single crystalsilicon minimizes contamination of substrates during plasma processing,and also wears smoothly during plasma processing, thereby minimizingparticles. Alternative materials that can be used for plasma-exposedsurface 13 of the upper electrode 12 including inner electrode member 24and the outer electrode member 26 are SiC or AlN, for example.

In the embodiment shown in the FIG. 1, the backing member 14 includes abacking plate 32 and a backing ring 34 extending around the periphery ofbacking plate 32. In the embodiment, the inner electrode member 24 isco-extensive with the backing plate 32, and the outer electrode member26 is co-extensive with the surrounding backing ring 34. However, inanother embodiment, the backing plate 32 can extend beyond the innerelectrode member 24 such that a single-piece backing member can be usedto support the inner electrode member 24 and the outer electrode member26. The inner electrode member 24 and the outer electrode member 26 arepreferably attached to the backing member 14 by a bonding material.

The backing plate 32 and backing ring 34 are preferably made of amaterial that is chemically compatible with process gases used forprocessing semiconductor substrates in the plasma processing chamber,and is electrically and thermally conductive. Exemplary suitablematerials that can be used to make the backing member 14 includegraphite, aluminum, aluminum alloys, and SiC.

The upper electrode 12 can be attached to the backing plate 32 and theoptional backing ring 34 with a suitable material, such as a thermallyand electrically conductive elastomeric bonding material thataccommodates thermal stresses, and transfers heat and electrical energybetween the upper electrode 12 and the backing plate 32 and backing ring34. The use of elastomers for bonding together surfaces of an electrodeassembly is described, for example, in commonly-owned U.S. Pat. No.6,073,577, which is incorporated herein by reference in its entirety.

For showerhead electrode assembly 10, process gas flows from the plenums31 formed in the thermal control plate 16 through gas passages 28 and 30in the upper electrode 12 and backing member 14, respectively, beforebeing injected into the space in a plasma processing chamber locatedbetween the upper electrode 12 and substrate support 18. The process gasenters gas passages 30 located in backing member 14 at the top surface15 of the backing member 14 at an entrance pressure (P_(INLET)) andexits passages 28 located in inner electrode member 24 at theplasma-exposed surface 13 at an exit pressure (P_(OUTLET)). Thedifference between the entrance pressure and exit pressure is the totalpressure drop (i.e., ΔP_(TOTAL)=P_(INLET)−P_(OUTLET)) across gaspassages 28 and 30.

Conductance, C, is the ability of the gas passages 28 and 30 to transmitgas therethorough. The conductance of a gas passage is determined by thesize and geometry of the gas passage. For example, the conductance of acylindrical gas passage increases with increasing diameter. Likewise,for a given gas passage diameter, the conductance of the gas passagedecreases as the length of the passage increases. The total ofconductance, C_(TOTAL), of gas passages 28 and 30 can be approximated asthe sum of the total conductance of gas passages 28 (C_(E)) in the upperelectrode 12 and the total conductance of gas passages 30 (C_(B)) in thebacking member 14 (i.e., C_(TOTAL)=C_(E)+C_(B)).

The overall gas throughput, Q, for gas passages 28 and 30 is determinedby the product of the total conductance of the gas passages 28 and 30and the total pressure drop (i.e.,Q=C_(TOTAL)ΔP_(TOTAL)=(C_(E)+C_(B))ΔP_(TOTAL)). However, because theinner electrode member 24 has a plasma-exposed surface 13, gas passages28 are subject to erosion, altering the geometry of each gas passage 28and raising the total conductance or C_(E) of gas passages 28. The totalpressure drop (ΔP_(TOTAL)) and the conductance of gas passages 30(C_(B)) remain substantially constant when gas passages 28 are eroded.However, any increase in conductance C_(E) due to erosion of the gaspassages 28 affects (i.e., increases) the overall gas throughput Q.

FIGS. 2A and 2B depict the occurrence of erosion of gas passages in anelectrode of an upper electrode. FIG. 2A is an enlarged view of aportion of a thermal control plate 16, which includes plenum 31 in fluidcommunication with axial gas passages 28 and 30 shown in FIG. 1. In FIG.2A, each gas passage 30 in the backing member 14 corresponds to a gaspassage 28 in the inner electrode. In other words, there is a one-to-onecorrespondence between gas passages 30 and 28. As a process gas flowsthrough gas passages 28 and 30 to supply the reaction chamber, there isa total pressure drop, across the gas passages 28, 30. For theembodiment shown in FIG. 2A, computer simulations have demonstrated thatabout 20% of the total pressure drop, ΔP_(TOTAL), occurs across gaspassages 30, located in backing member 14; the remaining portion of thetotal pressure drop (about 80% of total pressure drop) occurs across ingas passages 28. Gas passages 28 are located in inner electrode member24, which is subject to plasma erosion at plasma exposed surface 13.

As shown in FIG. 2B, the prolonged exposure of the inner electrodemember 24 to a plasma environment results in the erosion of the edges ofgas passage 28 (indicated by the dashed ovals) at the plasma-exposedsurface 13. The erosion of an axial cylindrical gas passage 28, resultsin widening of the gas passage 28 in the region closest to the plasma.This change in the geometry of the gas passage 28 increases itsconduction, C_(E), and thus changes the overall gas throughput throughgas passages 28. Furthermore, because a large percentage of the totalpressure drop occurs across gas passages 28, any change in theconductance C_(E) due to the erosion of gas passages 28 has a relativelygreater impact on the overall gas throughput Q of gas passages 28 and30. For example, assuming that ΔP_(E)=0.8 ΔP_(TOTAL) and ΔP_(B)=0.2ΔP_(TOTAL), the overall gas throughput Q=0.80(C_(E)ΔP_(TOTAL))+0.20(C_(B) ΔP_(TOTAL)). Accordingly, because of the greatercontribution of the C_(E) ΔP_(TOTAL) term to the overall throughput Q,any variations in the conductance, C_(E), of gas passage 28 due toplasma erosion have a greater impact in varying the gas throughput Qthan the C_(B) ΔP_(TOTAL) term. As a result, the inner electrode member24 must be periodically replaced.

It has been determined that one solution for maintaining a substantiallyuniform gas throughput Q through gas passages 28 and 30, even aftererosion of the gas passages 28, is to modify these gas flow paths toeffectively minimize the C_(E) ΔP_(TOTAL) contribution of the gaspassages 28 to the overall throughput Q, so that the effect of thevariation in C_(E) on Q as gas passages 28 erode due to plasma exposureis reduced. In one embodiment, the gas flow performance of the electrodeassembly is modified by shifting the distribution of ΔP_(TOTAL) suchthat a smaller percentage of the total pressure drop ΔP_(TOTAL) occursacross the eroded region of the gas passages 28. For example, gas flowpaths in the backing member 14 and the upper electrode 12 can bemodified such that a smaller percentage of the total pressure dropΔP_(TOTAL) occurs across gas passages 28 located in upper electrode 12.In other words, the contribution of the C_(E) ΔP_(TOTAL) term to theoverall throughput, Q, can be reduced by lowering the percentage of thetotal pressure drop ΔP_(TOTAL) across gas passages 28.

This reduction in the percentage of total pressure drop ΔP_(TOTAL)across the eroded region of the gas passages 28 can be achieved by oneor more of the following structural modifications: changing the totalnumber of gas passages 28 and 30; changing the shape and/or dimensionsof the gas passages 28 and/or 30; changing the ratio of gas passages 28in communication with respective gas passages 30; or through theaddition of one or more plenums of suitable geometries between thebacking member 14 and upper electrode 12. These structural features canbe optimized individually, or in combination, to achieve the desiredpercentage of the total pressure drop ΔP_(TOTAL) across gas passages 28during flow of gas through gas passages 28 and 30.

FIG. 3 illustrates an embodiment of a showerhead electrode assembly 10for a plasma processing apparatus, including a modification to thefeatures of the gas distribution passages 28 and 30. For thisembodiment, each axial gas passage 30 in the backing member 14corresponds to two gas passages 28 in the upper electrode 12 (e.g.,inner electrode member 24), with each gas passage 30 in fluidcommunication with a respective plenum 36 formed in a surface of thebacking member 14.

In alternative embodiments, each gas passage 30 in the backing member 14can correspond to more than two gas passages 28 in the inner electrode12, such as three or four gas passages.

FIGS. 4A and 4B illustrate a portion of the showerhead electrodeassembly shown in FIG. 3, depicting the thermal control plate 16, whichincludes plenum 31 in fluid communication with gas passages 30 and 28,with a plenum 36 in backing member 14. In other words, there is atwo-to-one correspondence between gas passages 28 and gas passage 30.The plenum 36 in backing member 14 serves to distribute gas uniformlyfrom gas passages 30 to gas passages 28. For this embodiment, backingmember 14 can be composed of aluminum and upper electrode 12 of silicon,for example.

FIG. 4B is a top view of the embodiment of a portion of the showerheadelectrode assembly shown in FIG. 4A in which backing member 14 and upperelectrode 12 are circular and the plenums 36 are radially-spaced annularchannels formed in a surface of backing member 14 facing upper electrode12 (thermal control plate 16 not shown for simplicity). Plenums 36 caneither be continuous or segmented. For example, in the embodiment, thediameter of gas passages 28 and 30 can range from 0.001 to 0.010 inches.The depth of the plenum 36 can range from 0.01 to 0.03 inches.

For the embodiment shown in FIGS. 4A and 4B, it has been demonstrated bycomputer simulation that about 78% of the total pressure drop occursacross gas passages 30, located in backing member 14; the remainingportion of the total pressure drop (about 22% of the total pressuredrop) occurs across gas passages 28. This is substantially opposite tothe contributions of the gas passages 28 and 30 to the total pressuredrop of the embodiment shown in FIG. 2A. Thus, in the embodiment shownin FIGS. 4A and 4B, because a much smaller percentage of the totalpressure drop (about 22% of total pressure drop) occurs across gaspassages 28, any change in the conductance C_(E) due to the erosion ofgas passage 28 has a smaller impact on overall gas throughput Q of gaspassages 28 and 30 (i.e., Q=0.22(C_(E) ΔP_(TOTAL))+0.78(C_(B)ΔP_(TOTAL))). As a result, because of the reduced contribution of C_(E)ΔP_(TOTAL) to the overall gas throughput Q, any variations in theconductance, C_(E), of gas passage 28 due to plasma erosion (such asshown in FIG. 2B) has a smaller impact in varying the throughput Q ascompared to such erosion of the gas passage 20 of the embodiment shownin FIG. 2A. For example, the ratio of C_(B) ΔP_(TOTAL) to C_(E)ΔP_(TOTAL) can range from about 3:1 to 5:1. As a result of producing amore consistent gas throughput, the silicon upper electrode 12 shown inFIGS. 4A and 4B can potentially provide a greater lifespan, although gaspassages 28 may experience erosion.

FIGS. 5A and 5B illustrate an alternative embodiment of the showerheadelectrode assembly, in which an additional plenum 38 is formed in thetop surface 15 of the backing member 14 between the plenum 31 and thebacking member 14. The plenum 38 is aligned with one or more gaspassages 30 and plenum 31 in the thermal control plate 16. As shown,plenum 38 has a smaller width than plenum 36. The dimensions of theplenum 38 can be optimized such that a smaller percentage of the totalpressure drop ΔP_(TOTAL) occurs in gas passages 28. For example, plenum38 can be an annular channel or groove.

FIG. 5B is a top view of the embodiment shown in FIG. 5A in whichbacking member 14 and upper electrode 12 are circular and the plenums36, 38 are radially-spaced annular channels formed on opposite surfacesof the backing member 14, facing upper electrode 12 and facing away fromupper electrode 12 (thermal control plate 16 not shown for simplicity).Plenums 36, 38 can either be continuous or segmented.

FIG. 6 is a top view of the upper electrode 12 of the embodiment inFIGS. 4A and 4B, illustrating an exemplary arrangement of gas passages30 and annular plenums 36 in backing member 14 (not shown) in relationto gas passages 28 in the upper electrode 12. Typically, the upperelectrode 12 is a circular plate and gas passages 28 and 30 extendaxially through the plate at different radial distances from the centeraxis C of the plate. In this embodiment, plenums 36A/36B can either becontinuous annular grooves, as shown in FIG. 4B or not extendedcontinuously for 360°, such as about 60° or about 90° (not shown).

FIG. 6 also illustrates two embodiments for different positionalconfigurations of gas passages 30 and 28. In one embodiment,corresponding to plenum 36A, gas passage 30 and two corresponding gaspassages 28 have different radial distances relative to the center C ofcircular upper electrode 12. In another embodiment, corresponding toplenum 36B, gas passage 30 and two corresponding gas passages 28 haveabout the same radial distance relative to the center C of the circularupper electrode 12, but all three gas passages have different angularpositions.

FIG. 7 is a three-dimensional perspective view of a portion of theembodiment of the showerhead electrode assembly of FIG. 6. FIG. 7 alsoillustrates the arrangement of gas passages 28, 30 and plenums 36A, 36B(backing member 14 not shown for simplicity).

The embodiment shown in FIG. 4, with a two-to-one correspondence betweengas passages 28 and gas passage 30, requires fewer gas passages 30 to beformed. Additionally, fluid communication between the gas passages 30and 28 can be achieved without precise alignment of the gas passages 30and 28. Instead, gas passages 28 only need to be aligned with acorresponding annular plenum 36.

In one embodiment of the showerhead electrode assembly, backing member14 is a metallic material, such as aluminum or an aluminum alloy.Metallic components are generally more cost effective and easier tomachine, in comparison to their non-metallic counterparts such asgraphite backing members. For some applications, metallic materialsprovide improved stability under extreme operating conditions andgenerate fewer particles than non-metallic materials, such a graphite.However, use of an aluminum backing member 14 may result in interactionsbetween certain process gases and the aluminum.

For example, fluorine-containing gas (e.g., CF₄, CHF₃) plasmas can beused in plasma process chambers for etching dielectrics or organicmaterials. The plasma produced from these gases is composed of partiallyionized fluorine gas, including ions, electrons, and other neutralspecies, such as radicals. However, aluminum chamber hardware, whenexposed to low-pressure, high-power, fluorine-containing gas plasma, canproduce aluminum fluoride (i.e., AlF_(X)) byproduct.

The embodiments of the showerhead electrode assembly shown in FIGS. 4A,4B and 5A, 5B is adapted to reduce the exposure of the aluminum backingmember 14 to such fluorine ions and/or radicals when fluorine-containingprocess gases are used. During plasma processing such process gases,fluorine ions or radicals may migrate through gas passages 28 and reactwith the aluminum backing member 14. Thus, the plenum 36 increases thediffusion length of an ion or radical (i.e., the distance from the upperend of the gas passage 28 to an aluminum surface of the backing member)as compared to the embodiment shown in FIG. 2A, reducing the probabilityof such ions or radicals interacting with the aluminum backing member14. In other words, the plenum 36 reduces the line of sight from ions orradicals in the plasma to an exposed aluminum surface.

In another embodiment, aluminum fluoride formation can be minimized bycoating the surface of the backing member 14 defining the plenum 36 witha coating to prevent reactions between the aluminum and fluorine. Forexample, the surface of the backing member 14 defining plenum 36 can beanodized, or coated with a suitable ceramic or polymer coating. Examplesof ceramic coatings include oxides (e.g., silicon oxide, aluminumoxide), nitrides or carbides. Examples of polymer coatings includepolytetrafluoroethylene (PTFE) or polyetheretherketone (PEEK).

FIG. 8 illustrates an additional embodiment of the showerhead electrodeassembly in which the erosion of gas passages 28 has lesser impact onoverall gas throughput Q of gas passages 28 and 30. As seen in FIG. 8,gas passages 28 include a first portion 40 and a second portion 42,wider than the first portion 38. The second portion 42 is adjacent toplasma-exposed surface 13. As process gas flows through gas passage 28,a greater fraction of the total pressure drop ΔP_(TOTAL) occurs acrossthe smaller first portion 40 than the second portion 42 of gas passages28. This reduction in the percentage of total pressure drop ΔP_(TOTAL)near the outlet end of the gas passage thus reduces the influence oferosion on the overall throughput Q flowing through gas distributionpassages 28 and 30.

While the invention has been described in detail with reference tospecific embodiments thereof, it will be apparent to those skilled inthe art that various changes and modifications can be made, andequivalents employed, without departing from the scope of the appendedclaims.

What is claimed is:
 1. A method of processing a semiconductor substratein a plasma processing apparatus including a showerhead electrodeassembly for a plasma processing apparatus, comprising a siliconelectrode with a plasma-exposed surface, the electrode having aplurality of axially extending first gas passages; a metallic backingmember attached to the electrode and having a plurality of axiallyextending second gas passages in fluid communication with the first gaspassages; one or more first plenums formed in the metallic backingmember and in fluid communication with the second gas passages; whereinwhen a process gas is flowed through the first and second gas passages atotal pressure drop is generated across the first and second gaspassages, the method comprising: placing a substrate on a substratesupport in a processing chamber of the plasma processing apparatus;introducing the process gas into the reaction chamber with theshowerhead electrode assembly so as to provide a ratio of total pressuredrop across the second gas passages to total pressure drop across thefirst gas passages of 3:1 to 5:1; generating a plasma from the processgas in the reaction chamber between the showerhead electrode assemblyand the substrate; and processing the substrate with the plasma.
 2. Themethod of claim 1, wherein the process gas is a fluorine-containing gasand the metallic backing member is composed of aluminum or an aluminumalloy.
 3. The method of claim 2, wherein surfaces of the metallicbacking member facing the electrode defining the one or more firstplenums are coated, wherein the coating is adapted to reduce aluminumfluoride formation.
 4. The method of claim 3, further comprising one ormore second plenums formed in a surface of the metallic backing memberfacing away from the silicon electrode, wherein each second plenum is influid communication with one or more second gas passages.
 5. The methodof claim 3, wherein surfaces of the backing member defining the firstplenums are coated with anodized aluminum, a ceramic, or polymericmaterial.
 6. The method of claim 1, wherein two or more of the first gaspassages are in fluid communication with one second gas passage.
 7. Themethod of claim 6, wherein the two or more first gas passages and theone second gas passage are located at different radial positionsrelative to a center of the respective electrode and backing member. 8.The method of claim 6, wherein the two or more first gas passages andone second gas passage are located at substantially the same radialpositions and different angular positions relative to a center of therespective electrode and metallic backing member.
 9. The method of claim1, wherein the electrode is composed of silicon, graphite, or siliconcarbide.
 10. The method of claim 1, wherein the one or more firstplenums are formed in a surface of the metallic backing member facingthe electrode.
 11. The method of claim 10, further comprising one ormore second plenums formed in a surface of the metallic backing memberfacing away from the electrode.
 12. The method of claim 11, wherein thefirst and second plenums are radially-spaced annular channels.
 13. Themethod of claim 11, wherein the first and second plenums are continuousor segmented.
 14. The method of claim 1, further comprising a thermalcontrol plate attached to the metallic backing member.
 15. The method ofclaim 14, wherein the first and second plenums are continuous orsegmented.
 16. The method of claim 15, further comprising one or morethird plenums formed in the thermal control plate, wherein each thirdplenum is in fluid communication with one or more second gas passages.17. The method of claim 1, comprising: obtaining the 3:1 to 5:1 ratio oftotal pressure drop across the second gas passages to the total pressuredrop across the first gas passages with a total number of first gaspassages, which is two to four times greater than a total number ofsecond gas passages.